Calibration Bath Research Articles

A distributed-temperature-sensing-based soil temperature profiler

Abstract. Storage change in heat in the soil is one of the main components of the energy balance and is essential in studying the land–atmosphere heat exchange. However, its measurement proves to be difficult due to (vertical) soil heterogeneity and sensors easily disturbing the soil. Improvements in the precision and resolution of distributed temperature sensing (DTS) equipment has resulted in its widespread use in geoscientific studies. Multiple studies have shown the added value of spatially distributed measurements of soil temperature and soil heat flux. However, due to the spatial resolution of DTS measurements (∼30 cm), soil temperature measurements with DTS have generally been restricted to (horizontal) spatially distributed measurements. This paper presents a device which allows high-resolution measurements of (vertical) soil temperature profiles by making use of a 3D-printed screw-like structure. A 50 cm tall probe is created from segments manufactured with fused-filament 3D printing and has a helical groove to guide and protect a fiber-optic (FO) cable. This configuration increases the effective DTS measurement resolution and will inhibit preferential flow along the probe. The probe was tested in the field, where the results were in agreement with the reference sensors. The high vertical resolution of the DTS-measured soil temperature allowed determination of the thermal diffusivity of the soil at a resolution of 2.5 cm, many times better than what is feasible using discrete probes. A future improvement in the design could be the use of integrated reference temperature probes, which would remove the need for DTS calibration baths. This could, in turn, support making the probes “plug and play” into the shelf instruments without the need to splice cables or experience in DTS setup design. The design can also support the integration of an electrical conductor into the probe and allow heat tracer experiments to derive both the heat capacity and the thermal conductivity over depth at high resolution.

Saturation vapour pressure measurements of refrigerant R1224yd(Z) from 274 K to 338 K

A new generation of refrigerant fluids, such as hydrofluorolefines (HFOs) and hydrochlorofluorolefines (HCFOs), has been recently introduced in order to increase the availability of zero-ODP (Ozone Depletion Potential) and low-GWP (Global Warming Potential) refrigerants. Among the HCFOs, the refrigerant HCFO-1224yd(Z) captured more attention due to its very low GWP (<1), negligible ODP (0.00023), non-flammability, low toxicity, and good chemical and thermal suitability. Due to its novelty, there is still a lack of thermodynamic property data about this refrigerant including, e.g. the saturation vapour pressure measurements that have been performed only in few experimental works, one of them used to develop the first equation of state in 2017. At INRIM, saturation vapour pressure measurements of the HCFO-1224yd(Z) were carried out in the temperature range from 274 K to 338 K, improving the measurement accuracy of previously available works. The experimental approach was based on a static method where the set-up consisted of a sample cell immersed into a temperature-controlled calibration bath with millikelvin stability, connected to a precision capacitance manometer with a measurement full scale of 0.5 MPa operating at a constant temperature of 373 K. The experimental work and the measurement results are reported together with a comprehensive uncertainty budget and a comparison against measurement data available from previous works.

Development of a calibration system for wafer-type temperature sensor

Wafer-type temperature sensors are widely used during semiconductor fabrication as integrated monitoring sensors for the real-time monitoring of wafer surface temperature, which affects the quality and yield of semiconductor devices. In recent years, various wafer-type temperature sensors have been developed according to the process type and temperature range for monitoring the temperature distribution. However, there is little research on the calibration system to ensure the accuracy and reliability of the developed wafer-type temperature sensor. In this study, we designed a system for the calibration of wafer-type temperature sensors. With this system, the traceability of the sensor can be guaranteed by calibrating the sensing characteristics, such as area uniformity, temperature stability, and accuracy. The conventional temperature calibration bath is calibrated by placing a wafer-type temperature sensor in a liquid-controlled system to have uniform temperature. This method is difficult to apply to the field because the wafer-type temperature sensor can be contaminated by liquids and can adversely affect semiconductor processes. For this reason, we used a heating chamber similar to chemical vapor deposition equipment for a calibration system. We evaluated the properties of the calibration system including the rising time (which means the time taken to reach the target temperature) and temperature stability according to the heat capacity. Moreover, we developed a measurement method that allows for the evaluation of the temperature of the wafer-type temperature sensor within 15 min at 150 °C. The calibration system and measurement method developed in this study are expected to improve the efficiency of semiconductor fabrication processes.

Calibration bath uncertainty in precision temperature measurements

Calibration baths are widely used in many laboratories worldwide for the calibration of thermometers by comparison. They come in different shapes and sizes and use different media (water, alcohol, silicon oil etc.), but a certain nonuniformity of theused media is always present. During precision temperature measurements, uniformity (homogeneity and temporal stability) of a calibration bath is of the utmost importance since its contribution to total measurement uncertainty is the largest. The temperature gradient can be described as a change of temperature at different positions inside the bath working volume. Temporal temperature stability depends on many factors such as: the bath temperature, control system, bath construction and the flow of the used media inside the bath, just to name a few. In this paper, we investigate uniformity of a cylindrical calibration bath. Homogeneity is determined by measurement of axial and radial temperature gradient inside the bath. Stability is examined by observation of the change in temperature reading during certain period of time. The obtained results are compared with manufacturer specification and can be used for determination of calibration bath contribution to total measurement uncertainty during calibration of thermometers.

Characterization of High Temperature Calibration Bath through Stability and Uniformity Tests with Data Acquisition using Standard Platinum Resistance Thermometer and Precision Multimeter

Calibration is one area in measurement where our country is still in the process of improvement. The absence of confirmed approaches and actions for identifying the features of calibration baths by numerous laboratories disallowed them to assess more the finest measurement ability and develop their accurateness. In recent advances in technology, there is now a way of providing the best measurement capability in temperature calibration by providing validated methods in characterizing calibration baths that contributes to the uncertainty of measurement of the calibrated thermometer and is therefore important to measure the extent of its contribution to the final uncertainty. This study adapted the process from obtainable and current methods. Platinum Resistance Thermometer (PRT-Isotech) and Standard Platinum Resistance Thermometer (SPRT-Hart) which are interfaced to Precision Digital Multimeters were used for measurements. Quite a few measurements were made at varying time durations implemented on several known positions in an orderly measurement form which enclosed the whole workspace. The uniformities and stabilities of the calibration bath were computed by getting the range of the Minimum Difference with the Maximum Difference taken at every established temperature point. Outcomes of measurements exhibited that the calibration bath stability at every temperature setting are steadier at lesser temperatures and have a tendency to rise as the temperature approaches 250oC. The liquid bath was further unvarying at lesser temperatures. The variances found were insignificantly distributed since the Standard Deviation ranged only from 0.005 to 0.013.

Design and Validation of the MBW Standard Humidity Generators

MBW Calibration AG (MBW) is the Designated Institute (DI) for humidity appointed by the Federal Institute of Metrology, METAS. MBW currently offers calibration and measurement capabilities (CMC) for frost/dew-point hygrometers by comparison with precision chilled-mirror transfer standards that have been calibrated using the primary standards of leading European National Metrology Institutes or DI. The design, construction and validation of two standard humidity generators to be used as the Swiss national standards for the primary realization of frost/dew-point temperature in the range from − 90 °C to + 95 °C are presented and discussed. The generators are operated as continuous flow “single-pressure” generators in the range from − 80 °C to − 10 °C with saturation over ice and from 0.5 °C to + 95 °C with saturation over water. Additionally, they are used in “two-pressure” mode for saturation over ice down to frost-point temperatures of − 90 °C and down to − 20 °C for saturation over water. The main saturators of both generators have been designed to fit in commercially available calibration baths with either ethanol or distilled water as the heat transfer fluid for saturator temperatures below and above 0 °C, respectively. Saturator temperature is measured using standard platinum resistance thermometers and a purpose-built precision thermometer. Pressure measurements are taken with gauge pressure transducers and a separate barometric sensor, to reduce the influence of the atmospheric pressure on the measurement of the pressure ratio and make full use of the correlation of pressure measurements and enhancement factors when operating in two-pressure mode. A totally automated pre-saturation and flow control system facilitates the calibration of state-of-the-art chilled-mirror transfer for standards without manual readjustment of the generated flowrate to ensure a constant volumetric flow at the conditions of the mirror. The uncertainty budget leading to the CMC for frost/dew-point temperature realization is presented in the context of the experimental validation performed. The results in the overlapping range of both generators are presented and used as further evidence of the saturation efficiency of both standards.

Dynamic Calibration for Permanent Distributed Temperature Sensing Networks

Distributed Temperature Sensing (DTS) is a technique that uses the interaction of laser pulses with silica to continuously sense temperature along the length of fiber-optic cables. The temporal and spatial resolution of DTS makes it an excellent technique for monitoring the performance of district-scale geothermal exchange borefields. A dynamic, double-ended calibration routine developed in response to site-specific challenges and constraints (i.e., more than 5 km, many splices, different fiber segments, and extended observation periods) is systematically presented and analyzed to provide novel insight on calibration considerations. Results show that different combinations of calibration baths may change calibration accuracy, and over determination in the calculation of calibration parameters provides greater accuracy. Fixing the $\gamma $ calibration parameter does not appreciably change accuracy but does provide a buffer against error from variations in calibration bath temperatures. Differential attenuation varied by up to 25% between discrete fiber sections and should be calculated for each array section to prevent errors generated from applying just one attenuation coefficient value for the entire fiber array. Furthermore, dynamically calculated differential attenuation may vary systematically with time and space. In a double-ended configuration, the consideration of whether the forward, reverse, or some combination of all light data is used will affect the robustness of the calibration over time. Each of these results may assist in thoughtful consideration of calibration design at future DTS installations facing similar challenges.

Development of a 300 L Calibration Bath for Oceanographic Thermometers

The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has been developing a 300 L calibration bath to calibrate 24 oceanographic thermometers (OT) simultaneously and thereby reduce the calibration work load necessary to service more than 180 OT every year. This study investigated characteristics of the developed 300 L calibration bath using a SBE 3plus thermometer produced by an OT manufacturer. We also used 11 thermistor thermometers that were calibrated to be traceable to the international temperature scale of 1990 (ITS-90) within 1 mK of standard uncertainty through collaboration of JAMSTEC and NMIJ/AIST. Results show that the time stability of temperature of the developed bath was within $$\pm 1 \,\hbox {mK}$$ . Furthermore, the temperature uniformity was $$\pm 1.3 \,\hbox {mK}$$ . The expanded uncertainty ( $$k=2$$ ) components for the characteristics of the developed 300 L calibration bath were estimated as 2.9 mK, which is much less than the value of 10 mK: the required specification for uncertainty of calibration for the OT. These results demonstrated the utility of this 300 L calibration bath as a device for use with a new calibration system.

Experimental Method for Determination of Self-Heating at the Point of Measurement

This paper presents a new experimental method and algorithm for the determination of self-heating of platinum resistance thermometer (PRT) when the temperature instability of medium of interest would prevent an accurate self-heating determination using standard methods. In temperature measurements performed by PRT, self-heating is one of the most common sources of error and arises from the increase in sensor temperature caused by the dissipation of electrical heat when measurement current is applied to the temperature sensing element. This increase depends mainly on the applied current and the thermal resistances between thermometer sensing element and the environment surrounding the thermometer. The method is used for determination of self-heating of a 100 $$\Omega $$ industrial PRT which is intended for measurement of air temperature inside the saturation chamber of the primary dew/frost point generator at the Laboratory for Process Measurement (HMI/FSB-LPM). Self-heating is first determined for conditions present during the comparison calibration of the thermometer, using the calibration bath. The measurements were then repeated with thermometer being placed in an air stream inside the saturation chamber. The experiment covers the temperature range between $$-65{\,}^{{\circ }}\hbox {C}$$ and $$10{\,}^{{\circ }}\hbox {C}$$ . Self-heating is determined for two different air velocities and two different vertical positions of PRT in relation to the chamber bottom.

Calibration of Air Thermometers in a Climatic Chamber and Liquid Baths

The paper deals with calibration of digital indication thermometers designed for air temperature measurements. These were calibrated by comparison with a reference thermometer in two different calibration media: in the air when a climatic chamber was used and in a liquid when a calibration bath was used. The main difference between the media was thermal conductivity, which was more than ten times lower in the air. To compare both methods, we calibrated two sets of air thermometers twice. The first set comprised of small temperature loggers with an internal sensor, whereas the second set was a group of temperature probes connected to the measurement unit. The temperature ranges for the loggers were from 0 \(^{\circ }\)C to 40 \(^{\circ }\)C and for the probes from −20 \(^{\circ }\)C to 60 \(^{\circ }\)C. The results showed that the measured temperature errors were smaller in the bath as well as less scattered. Furthermore, the assigned measurement uncertainty was lower, if calibration was performed in a calibration bath, mostly due to better thermal homogeneity of a liquid compared to the air in a climatic chamber. Calibration shall assure traceability of a measurement equipment in such a way that a calibration procedure simulates an actual use of equipment; otherwise, the corrections and associated measurement uncertainty could be misleading. Therefore, when air thermometers, which are designed to measure a gaseous medium, are calibrated, they should be calibrated under similar conditions as they are used. The paper presents the differences in results of calibration of air thermometers under different calibration conditions.

Development and characterisation of a bath-based vertical blackbody cavity calibration source for the range −30 °C to 150 °C

Industrial use of Radiation Thermometers (RTs) is becoming increasingly common due to the perceived advantages and wide market availability. Blackbody Cavity Radiation Sources (BCRSs) are typically used for calibration of these instruments, and these cavities are oriented horizontally in most cases. For BCRSs based in thermal baths, this necessitates the use of custom-built baths with side openings. This paper presents a unique design of vertical bath-based BCRS that may be immersed in conventional calibration baths without modifications to the baths. The method, results, and analysis of an international comparison comparing this vertical BCRS, standard horizontal BCRSs, and a previous iteration of the vertical design of BCRS are also presented. The comparison was conducted through collaboration between the Laboratory of Metrology and Quality, Slovenia (LMK) and the National Standards Authority Ireland (NSAI), with the intention of evaluating the suitability of the vertical orientation for calibration work. Transfer pyrometers and Standard Platinum Resistance Thermometers (SPRTs) were used as comparison standards. The transfer pyrometers used have spectral sensitivity from 8μm to 14μm in this temperature range. It was found that the vertical orientation was comparable to within 0.25°Cthroughout the range to standard horizontal cavities. It was concluded that a vertical configuration is an economical alternative for calibration of RTs within the range assessed.

Zinc-Filled Multi-entrance Fixed Point

Comparison calibrations comprise by far the majority of all thermometer calibrations, and the improvement of the calibration speed and quality is of significant importance. In general, these calibrations are performed by comparing a thermometer under calibration with a reference thermometer, with both thermometers being placed in a zone of controlled temperature that is in thermal equilibrium. The time invariance of metrological characteristics of temperature regulated zones (gradients, stability, repeatability) adds a significant contribution to the total calibration uncertainty. To improve spatial thermal homogeneity and temporal stability of ordinary equalizing blocks (commonly used in combination with calibration baths and furnaces), a new concept is developed under the name multi-entrance fixed point (MEFP). With the MEFP, performances are improved by utilizing a phase transition of matter, similar to the fixed-point cells that are used for calibration of thermometers at the primary level. While fixed-point cells represent temperature standards alone, the MEFP concept requires the use of a standard thermometer as the calibration reference. This paper discusses the advantages, design possibilities, and performance of the concept and gives results of the investigation of the zinc-filled MEFP cell. The performance of the new MEFP cell was examined by calibrating a thermometer against a standard. Two calibrations were performed in the same furnace, first using an MEFP cell as an ordinary equalizing block (without a phase transition) and then at the melt plateau of zinc. The investigation results confirmed the usefulness of the MEFP concept, as the uncertainty contributions from temperature gradients and the stability of the furnace were decreased by an order of magnitude, in comparison to the ordinary equalizing block.

A large aperture blackbody bath for calibration of thermal imagers

Thermal imagers are becoming widely used instruments for noncontact absolute temperature measurements as opposed to previous practice when they were mostly used to measure temperature differences. To assure accurate and reliable absolute temperature measurements, a calibration procedure including evaluation of the thermal imager entire field of view (FOV) is needed. The challenge was to construct a large aperture blackbody, covering the complete FOV of the thermal imager and having better stability and non-uniformity than the thermal sensitivity of the imager. The blackbody calibration bath was designed on hypothesis analogous to the multi zone furnace, where the role of electrical heaters was superseded by electrically controlled valves. The experimental work showed that the designed system enables traceable calibration of thermal imagers in the temperature range from 10 to 70 °C with the expanded uncertainty of 0.2 °C, while further investigations are needed to cover ranges beyond that.

Calibration and performance assessment of a temperature sensor prototype using a 1-point calibration procedure

This study aims to assess the relevance of 1-point calibration procedure, within the framework of the development of a new telemetric temperature sensor. The criteria used for performance assessment were the level of accuracy, and the time of inertia of the temperature sensor prototype (TSP) tested. First, the stability of the calibration bath was assessed. Then, the accuracy of 16 prototypes was evaluated for 7 target temperatures (ranging from 29 °C to 45 °C). Finally, the inertia of TSP response was evaluated while increasing and decreasing the bath temperature. The difference between prototype and target temperature increases as bath temperature moves away from 37 °C; however, the accuracy of the sensor conforms to applicable standards. Most TSP remain in the range of ±0.2 °C for each temperature level tested, but a linear, decreasing slope is observed; prototypes underestimate high temperatures and overestimate low temperatures. Data from time of inertia assessment show that probes were within the range of ±0.2 °C from the target temperature with a maximal delay of 150 s which satisfy standard norms. However, results indicate that a 1-point calibration procedure of the sensors appears non optimal, a 2-point calibration procedure should be performed to avoid the observed temperature data slope.

The Immersion Characteristics of Industrial PRTs

Immersion effects are one of the most significant sources of error in the use of industrial platinum resistance thermometers (IPRTs). This article combines the development of a mathematical model of immersion error and experimental measurements of the immersion characteristics of a range of IPRTs immersed in different fluids at different temperatures. The mathematical model relates the relative temperature error in the thermometer indication to two exponential terms, with one of the 1/e decay lengths three times the other. The experimental results show that both of the exponential terms are important for shallow immersion, but one is sufficient for long immersion. The decay length for the thermometers depends on the diameter of the probe and on the thermal environment into which it is immersed. For the thermometers evaluated here in mineral oil, silicon oil, and molten salt, the 1/e decay length is about three to four times the diameter of the thermometers. A simple rule of thumb for ensuring adequate immersion in calibration baths is developed.

GENERAL PROCEDURE FOR UNCERTAINTY EVALUATION OF A TEMPERATURE CALIBRATION BATH

ABSTRACT The objective of this paper is to develop a general procedure for evaluation of various temperature calibration baths or furnaces. Since the uncertainties of transfer measurement and temperature standards, such as standard platinum resistant thermometers in particular, are decreasing, metrological characteristics of calibration baths and furnaces are becoming a limiting factor in temperature calibration by comparison. Due to the fact that a time invariance of metrological characteristics of calibration baths (gradients, stability, repeatability, etc.) represents a major contribution to the total calibration uncertainty, the exact knowledge about a calibration bath is of an utmost importance. A general procedure for baths with different types of a media is proposed, which enables continuous monitoring of bath parameters in order to evaluate uncertainty contributions to a measured (calibrated) thermometer. There is no ideal calibration system, i.e., without inhomogeneity or gradient. Basic gradients that could be observed in calibration baths are vertical and horizontal gradients. They may also be recognized as axial and radial gradients in a case where cylindrical equalizing blocks are used inside a calibration bath. For the proper use of such a system in a process of calibration and measurement, gradients should be precisely determined. Gradients appear as a change of a temperature reading of a thermometer due to a change of its position inside a calibration bath. By this method, a realistic uncertainty contribution could be defined as opposed to under-or overestimated values of uncertainties, thus enabling an optimal use of a particular calibration bath. Examples of real data are taken from cryostat measurements. Otherwise, seven other temperature calibration baths and furnaces in the laboratory have been evaluated in the same way.

Analysis and Modeling of Equalizing Blocks

This paper deals with analysis and modelling of equalizing blocks in calibration baths for temperature calibration by comparison. The main purpose of the paper is to define, objectively, the parameters of the equalizing block that is usually designed on the basis of experimental data. Although heat transfer problems have been thoroughly analysed, a problem of determining optimal dimensions and shapes of equalizing blocks has not been completely discussed. An attempt was made to generalise a model of the equalizing block in order to optimise the configuration of various blocks in different calibration systems. For optimisation of the equalizing block, electrical modelling of calibration baths was analysed by using a simulation program. Theoretical results were verified by measurements in two highly stable liquid baths in the temperature range from −80°C to +290°C.

Reduction of uncertainties in temperature calibrations by comparison

The objective of this design note is to discuss and define the total uncertainty in temperature calibrations by comparison, by analysing most of the likely error sources. As a result of the proposed and developed uncertainty analysis, further reductions of uncertainties could be realized if/when better equipment becomes available. The analysis is performed as a case study using state-of-the-art calibration equipment described in the design note. This equipment is located in the authors' own secondary temperature calibration laboratory. Accreditation for this laboratory has been granted through The Dutch Council of Accreditation (RVA) for calibrations in the temperature range -55 to C. In temperature calibrations by comparison the four main groups of uncertainties are the reproducibility, uncertainty of a reference thermometer, uncertainty of a calibration bath or a furnace and uncertainty of a measuring device. Special care is taken, using a thorough evaluation procedure, to ensure that the uncertainty contribution of the calibration bath or furnace is as low as possible. This is necessary because the total uncertainty assigned to an instrument under calibration is larger than the largest individual uncertainty contribution. In temperature calibrations the largest uncertainty is most likely to be the uncertainty of the calibration bath or a furnace. Therefore this uncertainty typically represents the lowest limit for further reduction of the total uncertainty of the calibration process. The analysis performed allows optimal use of temperature calibration equipment for calibration of thermometers by comparison. In this way most practical calibration needs are satisfied in a more economical way than by using substantially more expensive fixed point calibrations.

Analysis of equalising blocks in calibration of thermometers

The paper deals with the analysis of equalising blocks in calibration of thermometers. The main purpose of the paper is to define objectively the parameters of the equalising block usually designed on the basis of experimental data. Although heat transfer problems have been thoroughly analysed, the problem of determining optimal dimensions and shapes of equalising blocks has not been completely discussed. An attempt is made to generalise the model of the equalising block in order to optimise the configuration of various blocks in different calibration systems. For the optimisation of the equalising block, an electrical model of the calibration baths was analysed by using a simulation program. Theoretical results have been verified by measurements in two highly stable liquid baths in the temperature range from −80°C to +290°C.

On-cruise calibration of XBT probes

A procedure is described to calibrate XBT probes at one temperature point immediately before launching. The individual XBT temperature offsets are determined by using a stirred calibration bath and a stable, accurate reference thermometer designed for field use. The proposed calibration procedure increases XBT temperature accuracy considerably, while it hardly raises time and effort spent on standard XBT drops. The performance of the system in the field and in the laboratory is described. It is shown that the achieved temperature accuracy is about 0.03 K, and is basically limited by the used reference thermometer.